1. Field of the Invention
The present invention relates to a spark plug used for an internal combustion engine. More specifically, the present invention relates to the spark plug having a screw diameter smaller than or equal to 12 mm (i.e., 10 mm or 12 mm).
2. Description of the Related Art
The present invention relates to a spark plug for an internal combustion engine. Especially, the present invention relates to the spark plug having a screw diameter smaller than or equal to 12 mm (for example, 10 mm, 12 mm and the like).
Conventionally, a spark plug having the following construction is well known:
A through hole having a first end side and a second end side (opposite to the first end side) is formed in a direction of an axial line of an insulator. A center electrode is inserted for fixation into the first end side of the through hole, while a terminal metal fitting is inserted for fixation into the second end side of the through hole. In the through hole, a resistor is interposed between the center electrode and the terminal metal fitting.
The resistor is made of a glass mixed with a conductive material (such as carbon black, metal and the like), where blending (proportion) of the metal is not considerably high. Thus, joining the resistor directly to the metals {that is, the center electrode and the terminal metal fitting} is very often of difficulty. Therefore, a conductive glass seal layer which is made of a mixture of metal (in plenty) and glass is conventionally interposed between the resistor and the center electrode, and between the resistor and the terminal metal fitting, for improving joint strength.
The conventional spark plug comprising the above resistor is manufactured in the following steps:
1) Insert the center electrode into the through hole of the insulator, for fixation.
2) Fill the through hole with conductive glass powder.
3) Fill raw material powder of a composition of the resistor.
4) Furthermore, fill the through hole with the conductive glass powder.
5) Then, insert the terminal metal fitting into the through hole, to thereby make an assembly.
As a result, the through hole of the insulator is allowed to have the following inner construction in a form of an accumulation sequentially from the center electrode""s side:
a) a first conductive glass powder layer,
b) a powder layer of the composition of the resistor, and
c) a second conductive glass powder layer.
In the thus obtained state, the assembly is conveyed to a heating furnace and heated at a glass softening point or over. Thereafter, the terminal metal fitting is pushed (press fitted) from a side opposite to the center electrode""s side into the through hole in the direction of the axial line, to thereby compress the first conductive glass powder layer, the powder layer of the composition of the resistor, and the second conductive glass powder layer.
The above compression forms, respectively, a first conductive glass seal layer {on the center electrode""s side}, the resistor, and a second conductive glass seal layer {on the terminal metal fitting"" side}. In other words, this construction allows the center electrode and the terminal metal fitting to join to the resistor, respectively, by way of the first conductive glass seal layer and the second conductive glass seal layer. The assembly having the thus obtained insulator, the center electrode and the terminal metal fitting is received for fixation in a metal shell which is shaped substantially into a tube.
Recently, preference for a smaller spark plug is likely to increase. To meet this preference, the spark plug is earnestly preferred to have its screw section""s diameter smaller than or equal to 12 mm (such as M12, M10 and the like under ISO 2705, where ISO stands for International Standardization Organization). The small spark plug, however, has such a restriction that the material used for the terminal metal fitting is limited due to difficulty of the above glass sealing steps. With this, the small spark plug cannot feature as high quality as a large spark plug {for example, a spark plug having its screw section""s diameter larger than or equal to 14 mm} can.
In the above glass sealing steps, the maximum heating temperature is generally about 900xc2x0 C. For the following cause, it is preferable to use the terminal metal fitting made of a steel product that is unlikely to get softened in the above heating temperature of about 900xc2x0 C.:
When the spark plug having the terminal metal fitting that is insufficient in hardness is mounted to the internal combustion engine, the terminal metal fitting may cause a wear due to a friction with a mouth piece of a plug cap. The thus caused wear may lead to failures such as flash over (attributable to wear powder), increased contact resistance (between the terminal section and the mouth piece of the plug cap), and the like.
For the terminal metal fitting, the spark plug having the large screw section""s diameter can use the steel product that is unlikely to get softened in the glass sealing step. However, such steel product (as is) cannot necessarily be used for the terminal metal fitting of the spark plug having the small screw section""s diameter.
More specifically, using the hard steel product for the terminal metal fitting of the spark plug having the small screw section""s diameter is, as the case may be, responsible for possible cracks which may be caused to the insulator in the glass sealing step.
The insulator itself of the spark plug having the small screw section""s diameter is thin. Such spark plug is, as a matter of course, low in strength. Using the hard steel product for the terminal metal fitting of the spark plug having the small screw section""s diameter cannot relax an excessive stress attributable to a deflection {of the terminal metal fitting} which may be caused when the terminal metal fitting is press fitted into the insulator. As a result, the excessive stress (beyond upper limit) is applied to the insulator.
On the other hand, the terminal metal fitting that is too diminished in size (diameter) for relaxing the stress applied to the insulator may cause insufficient glass sealing, resulting in failures such as low joint strength, increased contact resistance and the like.
The above description concerning the spark plug having the small screw section""s diameter ends up as what is called a dilemma.
It is an object of the present invention to provide a spark plug which is unlikely to cause wear to a terminal metal fitting, and which has an insulator, the terminal metal fitting and a center electrode that are connected securely. Especially, the present invention relates to the spark plug having a screw section diameter smaller than or equal to 12 mm.
More specifically, the spark plug of the present invention has the screw nominated by M12 (screw diameter 12 mm) and M10 (screw diameter 10 mm) under ISO 2705, where ISO stands for International Standardization Organization. Fluctuation in dimension is allowed within a range specified by the ISO 2705.
According to a first aspect of the present invention, there is provided a spark plug, comprising: a metal shell defining a front side having an outer periphery formed with a screw section which has a nominal size smaller than or equal to M12; a center electrode; a conductive seal layer; a terminal metal fitting fixed to the center electrode by way of the conductive seal layer; and an insulator formed with a through hole which extends in a direction of an axial line of the insulator and houses the center electrode, the conductive seal layer and the terminal metal fitting. The terminal metal fitting has a Vickers hardness in a range from 150 Hv to 300 Hv. The terminal metal fitting includes: a front end section having an outer diameter, and embedded in the conductive seal layer, and a small diameter section disposed on a rear side of the front end section and having an outer diameter which is smaller than the outer diameter of the front end section. The insulator has an outer periphery where the metal shell is disposed. The insulator defines the following sides in the direction of the axial line of the insulator: i) a front side where the center electrode is disposed, and ii) a rear side where the terminal metal fitting is disposed. The through hole has an inner diameter which is different from the outer diameter of the small diameter section of the terminal metal fitting in a range from 1.0 mm to 1.4 mm.
According to a second aspect of the present invention, there is provided a spark plug, comprising: a metal shell; a center electrode; a conductive seal layer; a terminal metal fitting fixed to the center electrode by way of the conductive seal layer; and an insulator formed with a through hole which extends in a direction of an axial line of the insulator and houses the center electrode, the conductive seal layer and the terminal metal fitting. The terminal metal fitting includes: a front end section having an outer diameter, and embedded in the conductive seal layer, and a small diameter section disposed on a rear side of the front end section and having an outer diameter which is smaller than the outer diameter of the front end section in a range from 0.3 mm to 0.7 mm. The small diameter section defining a front side which is partly embedded in the conductive seal layer. The insulator has an outer periphery where the metal shell is disposed. The insulator defines the following sides in the direction of the axial line of the insulator: i) a front side where the center electrode is disposed, and ii) a rear side where the terminal metal fitting is disposed.
The terminal metal fitting according to the first aspect and the second aspect of the present invention can keep the Vickers hardness in the range from 150 Hv to 300 Hv even after glass sealing steps. Thereby, even when engine vibration may cause a friction between a mouth piece {of a plug cap} and a terminal section {of the terminal metal fitting}, the terminal section is unlikely to get worn. Therefore, conductivity can be secured for a long time, thus making the spark plug highly reliable.
The Vickers hardness lower than 150 Hv may not bring about high wear resistance. Therefore, using the spark plug for a long time may cause the wear, thus leading to failures such as increased contact resistance {between the terminal section and the mouth piece of the plug cap}, flash over attributable to wear powder, and the like.
On the other hand, the Vickers hardness higher than 300 Hv may extremely limit material usable for the terminal metal fitting. Moreover, in this case, too high a hardness (or rigidity) of the terminal metal fitting may be responsible for the following failures:
When the engine vibration and the like may apply a stress to the terminal metal fitting, the thus applied stress cannot be dispersed effectively, resulting in concentration of the stress in a specific section of an insulator. The thus concentrated stress may break the insulator.
Herein, the Vickers hardness of the terminal metal fitting is defined as a value measured in the terminal section projecting rearward from the insulator.
The spark plug has the construction in which the outer diameter {of the small diameter section} and the inner diameter {of the through hole of the insulator} is in the range from 1.0 mm to 1.4 mm, thus making the spark plug small in screw section.
Generally, manufacturing the spark plug having the above construction uses the glass sealing steps.
In the glass sealing steps, the insulator is heated, while the terminal metal fitting is press fitted into the through hole of the insulator so as to compress a conductive glass powder layer and the like accumulated in the through hole. Press fitting the terminal metal fitting is carried out by means of a special equipment at a predetermined stroke. The terminal metal fitting featuring a xe2x80x9cproper softnessxe2x80x9d may elastically deform or plastically deform, thereby relaxing excessive stress which may be applied to the conductive glass powder layer and the insulator. On the contrary, allowing the terminal section to have the Viekers hardness of 150 Hv or over renders entire part of the terminal metal fitting to have the Vickers hardness of 150 Hv or over, which is not flexible.
Therefore, the small diameter section {of the terminal metal fitting} is so adjusted to have the outer diameter that is different from the inner diameter {of the through hole of the insulator} in the range from 1.0 mm to 1.4 mm. The thus adjusted difference can contribute to relaxation of the stress, by allowing the terminal metal fitting (even if high in hardness) to properly deflect toward the small diameter section.
The difference smaller than 1.0 mm cannot secure enough space for the terminal metal fitting to deflect in the glass sealing steps, thus failing to relax the excessive stress.
On the contrary, the difference larger than 1.4 mm may be responsible for an excessive space between the insulator and the terminal metal fitting, allowing the terminal metal fitting to excessively deflect toward the small diameter section, resulting in an insufficient joint strength in the glass sealing steps due to short pressure. Herein, the joint strength is a strength for joining the insulator and the terminal metal fitting.
Especially, the spark plug with the screw section smaller than or equal to 12 mm (M12) has a difficulty in increasing thickness of the insulator for improving strength of the insulator. Therefore, preventing cracks which may be caused in the glass sealing steps may be achieved by modification of the terminal metal fitting.
In sum, the spark plug according to the first aspect and the second aspect of the present invention can eliminate need for redesigning the insulator and secure glass sealing steps. Moreover, the spark plug according to the first aspect and the second aspect of the present invention can render the terminal metal fitting to securely join with the center electrode by way of the conductive seal material, contributing to failure-free (failures such as increased contact resistance) and small size {of the spark plug}.
The other objects and features of the present invention will become understood from the following description with reference to the accompanying drawings.